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Lithium Polymer batteries

Discussion in 'Alternative Energy' started by tcmtech, Mar 11, 2009.

  1. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    Any one got some real world experiance with li-po's big and small?

    What are the typical Ah VS size densities?
    Practical using tips? charge and discharge rates per ah or however they rate this.
    pros and cons?
    real life cost?

    You know real life operation peramiters and hands on experiance.

    My brother uses the small ones in RC stuff and the high capacity ones he has are pushing a power to size density about 12:1 over lead acid storage batterys. And supposedly 10x output amps over their Ah rating.

    5Ah li-po holds up at 50 amps draw. So he says. is this true or reasonable?
    is the 12:1 power ratio close on the high density ones?

    What goes into making one? they look simple enough.
    Can a person buy the sheets of whatever they are built out of?
    If so where and how much?
     
  2. dknguyen

    dknguyen Well-Known Member

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    Applications that use lead-acid batteries or Lipos usually cannot use the other for reasons of weight (advantage Lipo), power/energy (advantage Lipo), ruggedness (advantage Lead-Acid), or cost (advantage Lead-Acid). And there are very clear winners for each of these categories- the differences are significant enough that you almost can't swap one for the other.

    FOr the RC HObby market they tend to max out at 5000mAh capacities at 25.2V nominal (6 series cells), though you can find single packs with larger capacities and/or more series connected cells.

    Size densities? ??? Volumetric Energy Density? Weight Energy Density? Power Energy Density? What exactly? Size is far too vague...but Lipo beats lead-acid on all counts by a lot. You'll learn a lot more by browsing RC hobby websites for Lipo batteries which almost always give the volumetric dimensions and weight along with obvious data of voltage and capacity.

    Use a charger specifically designed to charge Lipos only. DO NOT overcharge or overdischarge, or discharge at excessive rates. THey rate discharge rates by using C-ratings. A C-rating is an output current that has been normalized to the capacity of the battery. So a battery with a total capacity of 10,000mAh (10Ah) that is beign discharged at a rate of 1C means you are drawing 10,000mA (10A) from it. A rate of 2C is 20A, and a rate of 15C, is 150A.

    Manufacturers get the maximum C-rating of a battery either by considering the minimum volage drop or by heat issues...which they use is not very clearly between manufacturers. Heat is obviously better...but if they use how well the battery can maintain it's voltage under high discharge, it doesn't mean the battery isn't overheating (and going to explode).

    Pros: Lightweight, high discharge rates, high energy density, no memory effect, fast charge time (relative to older technologies, there are much faster ones out there now like A123 cells)

    Con: Expensive, delicate, not tolerant of abuse (ie. overdischarge, overcharge, excessive discharge rates, catastrophic and dangerous failure modes

    For cost, just Google for Lipo prices in the RC market. No one is going to list every representative combination of voltage, capacity, and discharge here with the price. That said $10 per 1000mAh per cell (3.6V nominal) +/- 20% is a fairly good approximation for mainstream brands. However, note factory direct Lipos from China which which be 1/3rd of the price (and though it can be hit or miss, some of them have a much much higher performance:cost ratio tha mainstream brands).

    Drawing 50 amps from a 5Ah Lipo equates to a discharge rate of 10C (ie. a current draw 10x the capacity of the battery...and drop the hours at the end of the units). 10C is a conservative maximum rating for Lipos. THere are lipos with discharge ratings of 15C, 20C, 25C, and 30C...though you don't want to run them continuously at these ratings, but 10C is just fine.

    Again...power ratio is vague...power per weight? per volume? What do you mean by close? A single Lipo Cell (~3.6V) can discharge current of a rate of 10C-20C-30C (though you wouldn't want to do that for too long since it drastically shortens the useful life of the battery before trashing it), so let's be conservative and say 10C continuously. What C-rating does a lead acid batteyr have? C-rating is normalized to capacity so you can probably compare directly what way, and take into account that a Lipo cell has 3x the voltage of a Lead-Acid cell.

    What makes you think they are simple? All batteries look simple...two electrodes and an electrolyte... Solid state devices like electronic components and batteries are not like mechanical devices where if they look simple, then they tend to be simple to build. For solid state devices all the functionality lies in the way the materials interact with each other. So though it may just look like a few chunks of material sitting next to each other, a hell of a lot of work goes into making those materials.

    In short, You can't build Lithium polymer batteries (think about it, can you build lead-acid batteries? or any kind of useful battery for that matter?). It would require very reactive pure Lithium that is dangerous and also degrades very easily so you would need special facilities to keep it safe and maintain quality. You'd also need to make the polymer electrolyte used, let alone find out what needs to go into it and where to get the ingredients (R&D department anyone?). And then you'd need a way to safely package something so voltatile. And every step of the way you seem to need some kind of exotic materials.
    http://www.electro-tech-online.com/custompdfs/2009/03/620Lithium20Polymer.pdf

    Even if you could, you probably wouldn't want to due to safety issues and cost. Like previously mentioned- catastrophic (ie. explosive) failure modes. THe manufacture of real batteries of any kind (not the potato or lemon kind) are one of those things that are far far out of reach of anyone without a factory. And batteries, like microchips are mass produced so they are really really cheap for what they are and you are going to spend way more money to make your own even if you had your own factory to make them in...unless you were making more batteries than they were.
     
    Last edited: Mar 11, 2009
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  3. tcmtech

    tcmtech Well-Known Member Most Helpful Member

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    Thank you! this was actualy very helpful.
    Agian what looked like something simple is actualy harder to do than aperance gave it.
    I never got told much about the discharge or recharge part of it before.

    I am aware of the interchangability problems though. I simply used li-po vs LA because the LA batteries are so common. I could have used any other common battery as a refrence though.
    I have done some reading on the internet about them but as you put it the numbers from one place to another vary alot. hense the reason I asked for real world experiance info.
    Thanks bunches! you were helpful!
     
  4. dave

    Dave New Member

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  5. Wefi01

    Wefi01 New Member

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    Im looking for Lipo manufacturers for RC uses. Can anyone suggest? I went through Thomasregistry but could not find outside of US.
     
  6. Sceadwian

    Sceadwian Banned

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    It'll get easier over the years tcm, most if not all current electric vehicles use lithium batteries. I hesitate to call them batteries though they're more like cell networks because of the complexity of charging and discharging that many cells in parallel+serial together. Charging a lithium cell can be as simple as applying a constant current of 1C until a cell voltage of 4.1 volts is attained then maintain constant 4.1 volts until the current drops to 1/10th of the 1C initial rate, cell temperature should be monitor because lithiums are 95-100% charge efficient, so if it ever gets warm, something is wrong. Pack ballancing during both charge and discharge that is a pain.

    If you want something that scales well but has some of the same attributes (and shortcomings) of lead acid batteries, take a look at
    Vanadium redox battery - Wikipedia, the free encyclopedia
    It's never going to beat batteries as a portable storage sollution, but without space and weight restraints capacity has almost no limits.
     
    Last edited: Jul 21, 2010

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